This invention relates generally to the field of storing radioactive elements and specifically to systems and methods of drying spent nuclear fuel in preparation for long-term storage in the “dry state.”
In the operation of nuclear reactors, hollow zircaloy tubes filled with enriched uranium, known as fuel assemblies, are burned up inside the nuclear reactor core. It is customary to remove these fuel assemblies from the reactor after their energy has been depleted down to a predetermined level. Upon depletion and subsequent removal, this spent nuclear fuel (“SNF”) is still highly radioactive and produces considerable heat, requiring that great care be taken in its subsequent packaging, transporting, and storing. Specifically, the SNF emits extremely dangerous neutrons and gamma photons. It is imperative that these neutrons and gamma photons be contained at all times subsequent to removal from the reactor core.
In defueling a nuclear reactor, it is common place to remove the SNF from the reactor and place the SNF under water, in what is generally known as a spent fuel pool or pond storage. The pool water facilitates cooling of the SNF and provides adequate radiation shielding. The SNF is stored in the pool for a period long enough to allow the decay of heat and radiation to a sufficiently low level to allow the SNF to be transported with safety. However, because of safety, space, and economic concerns, use of the pool alone is not satisfactory where the SNF needs to be stored for any considerable length of time. Thus, when long-term storage of SNF is required, it is standard practice in the nuclear industry to store the SNF in a dry state subsequent to a brief storage period in the spent fuel pool, i.e., storing the SNF in a dry inert gas atmosphere encased within a structure that provides adequate radiation shielding. One typical structure that is used to store SNF for long periods of time in the dry state is a storage cask.
Storage casks have a cavity adapted to receive a canister of SNF and are designed to be large, heavy structures made of steel, lead, concrete and an environmentally suitable hydrogenous material. However, because the focus in designing a storage cask is to provide adequate radiation shielding for the long-term storage of SNF, size and weight are often secondary considerations (if considered at all). As a result, the weight and size of storage casks often cause problems associated with lifting and handling. Typically, storage casks weigh more than 100 tons and have a height greater than 15 ft. A common problem associated with storage casks is that they are too heavy to be lifted by most nuclear power plant cranes. Another common problem is that storage casks are generally too large to be placed in spent fuel pools. Thus, in order to store SNF in a storage cask subsequent to being cooled in the pool, the SNF is transferred to a cask, removed from the pool, placed in a staging area, dewatered, dried, and transported to a storage facility. Adequate radiation shielding is needed throughout all stages of this transfer procedure.
As a result of the SNF's need for removal from the spent fuel pool and additional transportation to a storage cask, an open canister is typically submerged in the spent fuel pool. The SNF rods are then placed directly into the open canister while submerged in the water. However, even after sealing, the canister alone does not provide adequate containment of the SNF's radiation. A loaded canister cannot be removed or transported from the spent fuel pool without additional radiation shielding. Thus, apparatus that provide additional radiation shielding during the transport of the SNF is necessary. This additional radiation shielding is achieved by placing the SNF-loaded canisters in large cylindrical containers called transfer casks while still within the pool. Similar to storage casks, transfer casks have a cavity adapted to receive the canister of SNF and are designed to shield the environment from the radiation emitted by the SNF within.
In facilities utilizing transfer casks to transport loaded canisters, an empty canister is first placed into the cavity of an open transfer cask. The canister and transfer cask are then submerged in the spent fuel pool. Prior to cask storage, the SNF is removed from the reactor and placed in wet storage racks arrayed on the bottom of spent fuel pools. For dry storage, the SNF is transferred in the submerged canister that is flooded with water and within the transfer cask. The loaded canister is then fitted with its lid, enclosing the SNF and the water from the pool within. The loaded canister and transfer cask are then removed from the pool by a crane and set down in a staging area to prepare the SNF-loaded canister for long-term dry storage. In order for an SNF-loaded canister to be properly prepared for dry storage, the United States Nuclear Regulatory Commission (“N.R.C.”) requires that the SNF and interior of the canister be adequately dried before the canister is sealed and transferred to the storage cask. Specifically, N.R.C. regulations mandate that the vapor pressure (“vP”) within the canister be below 3 Torrs (1 Torr=1 mm Hg) before the canister is backfilled with an inert gas and sealed. Vapor pressure is the pressure of the vapor over a liquid at equilibrium, wherein equilibrium is defined as that condition where an equal number of molecules are transforming from the liquid phase to gas phase as there are molecules transforming from the gas phase to liquid phase. Requiring a low vP of 3 Torrs or less assures that an adequately low amount of moisture exists in the interior of the canister and on the SNF so that the SNF is sufficiently dry for long-term storage.
Currently, nuclear facilities comply with the N.R.C.'s 3 Torr or less vP requirement by performing a vacuum drying process. In performing this process, the bulk water that is within the canister is first drained from the canister. Once the bulk of the liquid water is drained, a vacuum system is coupled to the canister and activated so as to create a sub-atmospheric pressure condition within the canister. The sub-atmospheric condition within the canister facilitates evaporation of the remaining liquid water while the vacuum helps remove the water vapor. The vP within the canister is then measured by placing appropriate measuring instruments, such as vacuum gages, into the canister and taking direct measurements of the gaseous contents present therein. If necessary, this vacuum procedure is repeated until a vP of 3 Torrs or less is obtained. Once an acceptable vP is reached, the canister is backfilled with an inert gas and the canister is sealed. The transfer cask (with the canister therein) is then transported to a position above a storage cask and the SNF-loaded canister is lowered into the storage for long-term storage.
Current methods of satisfying the N.R.C.'s 3 Torrs or less vP requirement are potentially dangerous, operationally time consuming, prone to error, subjects the SNF rods to high temperatures, and costly. First, the intrusive nature of the direct vP measurement is dangerous because the canister contains highly radioactive SNF. Any time the canister must be physically breached, there is the danger of exposing the surrounding environment and the work personnel to radiation. Moreover, the creation of sub-atmospheric conditions in the canister requires expensive vacuum equipment and can cause complicated equipment problems. Finally, the operational durations for vacuum drying are unacceptably long as vacuum drying times on the order of days is quite common. The vacuum operation is prone to line freeze ups and ice formation inside canister which can give false readings to the instruments. Lowering of the canister pressure causes a progressive loss of the heat transfer medium (gas filling the gaps and open spaces in the canister) resulting in substantial elevation of temperature of heat producing SNF rods.